A circular cylindrical shaft which is subjected to a torque is influenced by a pure shear stress. This stress state can be expressed in terms of its principal stresses as a compressive stress and a tensile stress, directed perpendicularly thereto, of the same magnitude. The principal stress directions are inclined at .+-.45 degrees to a generatrix to the cylinder surface.
If, within a measurement range of the shaft, a rotationally symmetrical, homogeneous magnetizing field, that is an H-field, is created with the aid of a surrounding stationary exciting winding, an equally homogeneous magnetic flux density, that is, a B-field, is obtained in the shaft in non-loaded state. When the shaft is loaded, the field plot of the B-field is distorted, which can be detected with the aid of detection windings.
The state of the art as regards the constructive design of torque transducers is disclosed in a number of patent specifications and technical articles. Common to most of these solutions is that two zones are created in the magnetic material, with some form of anisotropy which causes the magnetic flux density to be deflected at an angle away from its natural direction in parallel with the generatrices to the cylinder surface of the transducer shaft. In one zone the principal direction of the anisotropy coincides with the principal stress direction which provides tensile stress. In the other zone, the principal direction coincides with the direction which provides compressive stress.
Because of the magnetoelastic effect, the zone reluctance will therefore decrease or increase, where the magnetic flux density has been deflected towards the tensile direction or the compressive direction in the case of positive magnetostriction. By finally measuring the difference in reluctance between these zones, a measure of the torque is obtained which has little sensitivity to axial forces or bending stress.
The reluctance difference between the zones is usually measured by creating, via a primary coil concentric with the shaft, a time-dependent H-field directed along the shaft and with equal amplitude in both zones. With the aid of two identical secondary coils, one across each zone, the difference in B-fields between the zones is measured. This is achieved in the simplest way by connecting the secondary coils in opposition in such a way that the induced voltages in the respective coil are subtracted from each other. By phase-sensitive rectification of the secondary signal obtained in this way, it is possible, in addition, to distinguish between torsional moments of different directions.
To create a high sensitivity to torsional moments, it is required that the anisotropy be sufficient, such that the difference between the zones becomes as great as possible. A measure of the anisotropy is the angle at which the magnetic field is deflected from the natural direction parallel to the generatrix to the cylinder surface of the transducer shaft because of the influence of the anisotropy. If this angle is 45 degrees in the zones, the anisotropy is maximal as the B-field is then directed along the principal stress directions of the transducer shaft loaded with the torsion.
Of the utmost importance is also to really maintain a complete rotational symmetry, both with respect to the mechanical stress configuration and the magnetic field configuration in order to prevent a signal variation which is only due to the transducer being rotated in relation to the reluctance-measuring part.
According to the state of the art, there are a number of methods of achieving anisotropy, a few of which will be described in the following.
SU 667836 describes a method in which the anisotropy is created purely geometrically in each zone by cutting slits in the surface of the shaft according to a specific pattern. This pattern consists of a number of mutually parallel lines directed at an angle of 45 degrees to a generatrix to the cylinder surface of the transducer shaft. However, this solution entails a limited anisotropy and hence also low sensitivity, since the magnetic field can "creep under" the slits unless these slits are made deep. If the slits are made deep, however, the stress level in the surface of the shaft, and hence also the sensitivity, will be lowered. In addition, the slits in the surface lead to greatly increased effective stresses in the bottom of the slits and therefore the shaft can only be loaded to a moderate extent before plastic yielding of the shaft material sets in, which in turn leads to hysteresis in the output signal of the transducer.
U.S. Pat. No. 4,823,620 describes the same embodiment as above with respect to the geometrical anisotropy, however with the addition that the surface is hardened or carburized for the purpose of reducing the hysteresis in the transducer.
SU 838448 describes a method in which an attempt has been made to increase the sensitivity by instead producing slits by knurling a similar pattern onto the shaft surface. In this way the anisotropy is increased by plastically deforming the material nearest the slit. This provides high residual stresses and hence a lower permeability along the bottom of the slits and, therefore, an anisotropy of a magnetic nature. The problem with yielding in the shaft material will probably be accentuated with such a solution although plastic machining per se increases the yield point.
In a torque transducer according to U.S. Pat. No. 4,506,554 anisotropy is obtained by using a sleeve of magnetoelastic material with cut-away slits in the principal stress directions. In this way the magnetic field can be prevented from "creeping under" the slits as above and a certain freedom is obtained in choosing a shaft material with other magnetic properties than the sleeve material. The latter material must primarily be chosen in view of the magnetic properties.
Another realization of the same kind is described in IEEE Trans Magn, Vol. MAG-22, No. 5, pp. 403-405, by Mohri et al. Here, a 100 micrometer thick "sleeve" with continuous slits on a stainless shaft is obtained by spraying onto the shaft droplets of a molten magnetoelastic alloy through a mask.
Other variants of the same method are described where strips of an amorphous magnetoelastic material are glued or otherwise applied to a magnetic or non-magnetic transducer shaft in the principal stress directions thereof. The problems with residual stresses, temperature drift, etc., are often awkward for these designs.
For transducers which are to be used for measuring torque which is always directed in one and the same direction, it is, in principle, sufficient to have one measuring zone on the transducer shaft. In other contexts and for special purposes, more than two measuring zones may also be used.
EP 0 270 122 B1 describes a "Magnetoelastic torque transducer" which is also based on the magnetoelastic principle and comprises in the usual manner excitation and sensing windings. The transducer shaft here has a ferromagnetic magnetoelastic region, selected from the group of materials which consists of iron-nickel martensite, hardenable and thermally hardened steels, which exhibit a substantially isotropic magnetostriction with an absolute amount of at least 5 ppm and containing from 0.05 to 0.75 per cent by weight carbon and a sufficient amount of an element selected from the group nickel, chromium, cobalt, titanium, aluminium, manganese, molybdenum, copper, boron, and combinations thereof to increase the magnetostriction value to at least 5 ppm in absolute amount. A form of anisotropy is here achieved by creating residual stresses in the material with the aid of cold-working, for example rotation or rolling.
As mentioned under "Technical Field" above, the invention is based on the use of material in the shaft of the transducer with a microstructure with at least two phases which are anisotropically distributed. It is commonly known that an example of such a material is a bar of ferritic-austenitic stainless steel. The bar manufacture is performed by greatly directed forming steps which give the bar a geometrically anisotropic microstructure in the form of axially directed long parallel streaks of ferrite and austenite.